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If you run at any given pace long enough, you will eventually get tired and slow down. Start running at 50 percent of your maximum sprint speed and you might last a few hours, assuming you’re fit. Start running at 75 percent of your maximum sprint speed and you’ll only last a few minutes.

Various changes occur in your body as you run toward fatigue. The specific changes depend on your speed. Some of these changes can be used to predict the point at which you’ll become exhausted. Blood lactate level, muscle glycogen level, breathing rate and core body temperature are among the variables that scientists can use to anticipate the onset of fatigue in different circumstances. But there’s one variable that predicts fatigue at all exercise intensities better than any other: perceived exertion, or how difficult the exercise effort feels.

Perceived exertion increases linearly throughout sustained exercise at any given intensity. Thus, scientists can use the rate of increase of a runner’s perception of exertion to accurately predict how much longer he or she will last.

Why is the intangible psychological variable of perceived exertion a better predictor of exercise fatigue than any physiological variable? Some scientists believe it’s because the feeling of fatigue is the real cause of fatigue. In other words, we always slow down when we feel exhausted because we feel exhausted.

Strong evidence in support of this idea comes from studies demonstrating that particular nutrients or drugs enhance exercise performance by acting on the brain in ways that reduce perceived effort. But in addition to proving that the feeling of fatigue is the cause of fatigue in exercise, these studies also reveal ways that everyday runners like you and me can enhance our race performance.

On the following pages are four nutrients and legal drugs that are proven to boost endurance capacity by making running feel easier.